1. Field of the Invention
The present invention generally relates to optical pickup devices and particularly to optical pickup devices used for magneto-optical disc apparatuses.
2. Description of the Background Art
In recent years there has been developed a magneto-optical disc reproduction apparatus capable of repeatedly recording and reproducing audio and video data, documents and other similar data. An optical pickup device is used as a main component of the magneto-optical disc reproduction apparatus and its miniaturization is considered important.
The present inventors have proposed a miniaturized optical pickup device in Japanese Patent Laying-Open No. 2001-034989.
With reference to FIG. 17, Japanese Patent Laying-Open No. 2001-034989 discloses an optical pickup device including a light source 103, a collimator lens 108 and an objective lens 109 collecting and passing light from light source 103 onto a magneto-optical (MO) disc 110, a photodetector 124 detecting light reflected from MO disc 110. This optical pickup device further includes a polarization separating prism 105 formed of anisotropic optical member 101 and anisotropic optical member 102, arranged on an optical path extending from light source 103 to collimator lens 108 and having an optical branching function to internally reflect light from light source 103 to guide it to collimator lens 108 and to transmit light reflected from MO disc 110.
Isotropic and anisotropic optical members 101 and 102 are arranged to have a boundary surface 105a with an inclination of 45xc2x0 to an optical axis 114 of reflected light.
Thus a light beam B1 from light source 103 is directed parallel to optical axis 114 and a light beam B2 reflected by a plane of reflection 101c is directed perpendicular to optical axis 114.
In this configuration, an isotropic optical member 102 is formed for example of LiNbO3, which is stable in electrochemistry, has a large difference in refraction index, and can be produced in a large amount at one time and thus available at low cost, and isotropic optical member 101 is for example TaFD30, a dense and inexpensive optical glass available from HOYA CORP.
Light transmitted through boundary surface 105a has aberration. Thus on photodetector 124 a spot has a geometry larger than the source light, as shown in FIG. 18. Light transmitted through boundary surface 105 is separated into an ordinary ray R1 and an extraordinary ray R2. Ordinary ray R1 provides a spot having a length of 80 xcexcm in a direction T1 and a length of 100 xcexcm in a direction orthogonal to direction T1. Extraordinary ray R2 provides a spot having a length of 60 xcexcm in both of a direction T2 and a direction orthogonal to direction T2. Herein, T1 and T2 are directions corresponding to a guide groove of the MO disc. A beam enlarged by aberration is detected by a light receiving portion 126 divided in four, to provide focus servo through astigmatism, and a beam is detected at a light receiving portion 125 divided in two, to provide tracking servo by a push-pull method (a 1-beam method).
Generally, if the 1-beam method is employed, shifting objective lens 109 causes unbalance of light, (hereinafter referred to as a xe2x80x9cradial offsetxe2x80x9d) on photodetector 125. As such, if the 1-beam method is implemented, the radial offset also needs to be prevented. However, to prevent the radial offset a high level of technique is required, and most of manufactures thus avoid adopting the 1-beam method. Thus the method is less prevalent in the market.
By contrast, a 3-beam method eliminates the necessity of accommodating the radial offset. As such, a simple technique is sufficient to provide stable tracking servo, which makes optical pickup devices more prevalent.
With reference to FIG. 19 is shown a spacing between a main beam and a subbeam in an optical pickup device of a typical 3-beam system. If the main beam and the subbeam are spaced by 17 xcexcm on an MO disc 131 in a tangential direction, the beams are spaced on a photodetector 132 by approximately 50 to 60 xcexcm, attributed to a constant of collimator lens 133 and that of objective lens 134.
From a different point of view the present inventors are also currently studying and developing a more miniaturized and highly integrated optical pickup device, as has already been proposed in Japanese Patent Laying-Open No. 2000-348374. Reference will now be made to FIGS. 20 and 21 to describe a configuration of the optical pickup device. This optical pickup device includes a stem 111, a semiconductor laser 103 provided on stem 111 to serve as a light source, a cap 113 covering stem 111, and an optically transmissive substrate 114 attached on cap 113. Furthermore the optical pickup device also includes a xc2xd wavelength plate 115 attached on optically transmissive substrate 114, an optical element 105 attached on xc2xd wavelength plate 115, and a collimator lens 108 and an objective lens 109 collecting on a magneto-optical recording medium 110 a beam of light emanating from laser diode 103. Furthermore the optical pickup device also includes a photodetector 124 arranged on stem 111 to detect light reflected from magneto-optical recording medium 110 and branched by optical element 105. On optically transmissive substrate 114 are arranged first and second diffraction elements 121 and 122.
A beam of light emanating from laser diode 103 passes through the second diffraction element 122 and is separated into transmitted light and three, positive and negative first-order diffracted beams of light. The light then passes through xc2xd wavelength plate 115 and it is reflected by optical element 105 at first and second surfaces 101c and 105a, passes through collimator lens 108 and objective lens 109 and is then collected on magneto-optical recording medium 110. Magneto-optical recording medium 110 provides a reflection of light, which beam is separated into an ordinary ray and an extraordinary ray at an angle of refraction determined by a ratio of a refraction index of the first member to an ordinary index of the third member, and a ratio of the refraction index of the first member to an extraordinary index of the third member. The rays then arrive at the first diffraction element 121 underlying optical member 105 and they are further separated into transmitted light and diffracted light and thus collected on photodetector 111.
Optical element 105 is formed of a first member 101 formed of an isotropic glass material and a third member 102 formed of an anisotropic material (a birefringent material). The first and third members 101 and 102 have therebetween a boundary surface serving as a polarization separating surface. A reflection of light from magneto-optical recording medium 110 that refracts at the second surface 105a has a wave aberration, which is compensated for by forming the optical element from materials so selected that an average of ordinary and extraordinary indexes of refraction of the birefrigent material forming the third member 102, and an index of refraction of the glass forming the first member 101 substantially match in value. For example, the glass material for the first member 101 may be LF5, a product of Schott Group with n of 1.5722, and the birefringent material for the third member may be lithium tetraborate having no of 1.605 and ne of 1.549.
The first diffraction element 121 is divided into first to third regions. Furthermore, photodetector 124 is structured as shown in FIG. 21. Light transmitted through the first diffraction element 121 is collected on each of photodetection portions 124e-124h. Light diffracted in the first diffraction element at the first region is collected on a boundary surface of photodetection portions 124c and 124d. Light diffracted at the second region is collected on photodetection portion 124a. Light diffracted at the third region is collected on photodetection portion 124b. 
From the intensity of light detected on each photodetection portion is obtained the following information: initially, (a) a difference between signals output from photodetection portions 124c and 124d can be calculated to obtain a focus error signal based on a knife edge method. Also, (b) a difference between signals output from photodetection portion 124g and 124h can be calculated to obtain a radial error signal based on a 3-beam method. Furthermore, (c) a difference between signals output from photodetection portions 124a and 124b can be calculated to obtain a so-called push-pull signal, which is used to detect a signal of an address meandering and thus recorded in a magneto-optical recording medium. Furthermore, a magneto-optical signal can be obtained by calculating a difference between signals output from photodetection portions 124e and 124f. 
In the optical pickup device as described above, a beam of light emanating from semiconductor laser 103 and following an optical path to arrive at magneto-optical recording medium 110, does not pass any other extra optical branching element than a polarizer prism and the second diffraction element 122. This ensures that light can be used efficiently. Furthermore, employing a single photodetector 124 to detect all of a magneto-optical signal, a focus error signal and a radial error signal, can reduce the area of the photodetector. Thus the optical pickup device can further be miniaturized and manufactured at a further reduced cost.
In the FIG. 17 optical pickup device, however, aberration results in a beam having a size no less than 60 xcexcm on photodetector 124, as shown in FIG. 18. Thus, while the device is advantageous for the push-pull method (1-beam method), it is not applicable without modification in the form of an optical pickup device in the 3-beam system, since in the 3-beam system a main beam and a subbeam are hardly arranged as appropriate.
Furthermore in the FIG. 20 optical pickup device the birefringent material for the third member, i.e., lithium tetraborate is expensive and it is also deliquescent and thus needs to be protected for example by a moisture-proof coating. This increases the cost of the entire optical pickup device. If the third member is formed of lithium niobate, a stable and inexpensive birefringent material, the optical pickup device can be manufactured at a low cost.
However, lithium niobate has high ordinary and extraordinary indexes no and ne of 2,258 and 2.178, respectively. Since glass material has an index of refraction of at most approximately two and wave aberration cannot be compensated for by selecting an average of ordinary and extraordinary indexes of refraction of lithium niobate and an index of refraction of glass serving as the first member, to substantially match each other in value. In other words, wave aberration cannot be compensated for for both of ordinary and extraordinary rays in the birefringent material. Thus for example if lithium niobate is combined with SF2, an inexpensive glass material produced by Schott Group having n of 1.635, then a problem would occur as described below: when light reflected from a magneto-optical recording medium refracts at the second surface, an angle of refraction determined by a ratio of a refraction index of glass to the ordinary index of lithium niobate and by a ratio of the refraction index of glass to the extraordinary index of lithium niobate, is increased and the reflected light thus would have a wave aberration. If wave aberration is introduced at the second surface 105a, light transmitted through the first diffraction element 121 and light diffracted thereby would both similarly have wave aberration. Thus, as shown in FIG. 22, in directions Y and Z a focal point has a positional displacement, and on the photodetector a beam of light is collected in the form of a spot distorted as shown in FIG. 23, and would thus be increased in size. This makes it difficult to design a beam arrangement, and furthermore an error introduced in fabricating an optical pickup unit, expansion and contraction of unit components that are attributed to changes in the environment thereof, and the like can result in a beam spot missing the photodetector. Consequently, the photodetector outputs a false signal. Thus, reliable signal reproduction cannot be achieved.
The present invention contemplates a miniaturized, 3-beam optical pickup device capable of detecting a stable signal if an optical element thereof is formed of an inexpensive material.
The present invention in one aspect provides an optical pickup device including: a light source; a lens arranged on an optical path extending from the light source to a magneto-optical recording medium; an optical element arranged on an optical path extending from the light source to the lens, and separating polarized light of light reflected by the magneto-optical recording medium; and a photodetector detecting light separated by the optical element. The optical element includes: a first member formed of an isotropic optical medium, receiving light from the light source and reflecting the received light to direct the reflected light to the magneto-optical recording medium; a second member formed of an isotropic optical medium and arranged adjacent to the first member, further passing the light reflected by the magneto-optical recording medium and having passed through the first member; and a third member formed of an anisotropic optical medium and arranged adjacent to the second member, separating via a boundary surface of the second and third members the light having passed through the second member, and directing the separated light to the photodetector.
The first and second members have therebetween a boundary surface branching light, and the second member formed of an isotropic optical medium and the third member formed of an anisotropic optical medium have therebetween a boundary surface separating polarized light of light reflected by a magneto-optical recording medium. Thus, light-branching and light separation can be provided independently and light after separation of polarization can have an aberration adjusted independently. This can provide an increased degree of freedom in design, such as reducing the size of a beam on a photodetector, to provide a miniaturized optical pickup device in a 3-beam system.
In the pickup device of the above one aspect preferably the first member is a prism having a cross section in a parallelogram having first parallel surfaces opposite to each other and second parallel surfaces opposite to each other each having a predetermined angle relative to the first parallel surface, one of the first parallel surfaces being in contact with the second member, one of the second parallel surfaces being arranged opposite to the light source, the other of the second parallel surfaces being arranged opposite to the lens.
As such the light source can be accommodated internal to a package to miniaturize the optical pickup device.
In the optical pickup device of the above first aspect still preferably the first and second members have the same index of refraction.
The first and second members having the same index of refraction can prevent reflected light transmitted through their boundary surface from having an aberration.
Still preferably the anisotropic optical medium is LiNbO3.
The anisotropic optical medium of LiNbO3 allows the optical pickup device to be produced at low cost.
In the optical pickup device of the above first aspect still preferably a ratio of an index of refraction of the isotropic optical medium to a larger one of indexes of refraction of the anisotropic optical medium, is at least approximately 0.77.
Since the ratio of the refraction index of the isotropic optical medium to the larger refraction index of LiNbO3 is no less than approximately 0.77, beam aberration can be reduced. Furthermore, if a component has a dimension tolerance, a fabrication tolerance and the like having an effect to change a beam""s position the beam can be prevented from missing a light receiving portion of a photodetector.
In the optical pickup device of the above first aspect still preferably the second and third members have the boundary surface with an angle xcex1xc2x0 relative to an optical axis of the reflected light to satisfy the following equation:
xcex1xc2x0=65xc2x15xc3x97(n/n1/0.77)
wherein n represents the index of refraction of the isotropic optical medium and n1 represents the larger one of indexes of refraction of the anisotropic optical medium.
We have confirmed in an experiment that if inclination xcex1 falls within a range of angle represented by the above expression a beam can be free of a significant aberration and furthermore if a component has a dimension tolerance, a fabrication tolerance and the like having an effect to change the beam""s position the beam can be prevented from missing a light receiving portion of a photodetector.
Still preferably the anisotropic optical medium is YVO4.
The anisotropic optical medium of YVO4 allows ordinary and extraordinary rays to be spaced wider to provide an increased degree of freedom in beam arrangement on a photodetector.
In the optical pickup device of the above first aspect still preferably a ratio of an index of refraction of the isotropic optical medium to a larger one of indexes of refraction of the anisotropic optical medium, is at least approximately 0.72.
Since the ratio of the refraction index of the isotropic optical medium to the larger refraction index of YVO4 is no less than approximately 0.72, beam aberration can be reduced. Furthermore, if a component has a dimension tolerance, a fabrication tolerance and the like having an effect to change a beam""s position the beam can be prevented from missing a light receiving portion of a photodetector.
In the optical pickup device of the above first aspect still preferably the second and third members have the boundary surface with an angle xcex1xc2x0 relative to an optical axis of the reflected light to satisfy the following equation:
xcex1xc2x0=67xc2x17xc3x97(n/n1/0.72)
wherein n represents the index of refraction of the isotropic optical medium and n1 represents the larger one of indexes of refraction of the anisotropic optical medium.
We have confirmed in an experiment that if inclination xcex1 falls within a range of angle represented by the above expression a beam can be free of a significant aberration and furthermore if a component has a dimension tolerance, a fabrication tolerance and the like having an effect to change the beam""s position the beam can be prevented from missing a light receiving portion of a photodetector.
The optical pickup device of the above first aspect still preferably further includes two xc2xd wavelength plates, one of the xc2xd wavelength plates being arranged between the light source and the boundary surface of the second and third members, the other of the xc2xd wavelength plates being arranged between the boundary surface of the second and third members and the lens.
Arranging two xc2xd wavelength plates allows the optical pickup device to be generally reduced in thickness, in addition to providing a function to direct readily reflectable, s-polarized light to a boundary surface of the first and second members and also allowing a magneto-optical recording medium to receive polarized light orthogonal to a guiding group.
The optical pickup device of the above first aspect can further include a diffraction element arranged between the optical element and the photodetector and having a hologram pattern compensating for a wave aberration introduced when the boundary surface of the second and third members refracts light.
With this configuration, (a) a polarization separating prism can correct wave aberration and (b) a diffraction element can also correct wave aberration. As such, if the polarization separating prism is formed of an inexpensive material a beam can be reduced in size and also arranged as desired. Thus furthermore reliable photodetection can be achieved.
In the optical pickup device of the above first aspect the hologram pattern corresponds to a locus of a point H on the diffraction element satisfying a relationship:
(LHxe2x88x92PH)xe2x80x2=nxcex
wherein L represents a point of light transmitted through a first diffraction element and collected, LH represents an optical path length between point H and a point L, P represents a point of light diffracted by the first diffraction element and collected on the photodetector, PH represents an optical path length between point P and point H, xcex represents a wavelength of a beam of light, n represents an integer, (LHxe2x88x92PH)xe2x80x2 represents a difference between optical path lengths LH and PH with a wave aberration at the boundary surface considered together with one of optical path lengths LH and PH.
This hologram pattern allows a diffraction element to set each beam""s arrangement, as desired, in a 3-beam method.
The present invention in a second aspect provides an optical pickup device including: a light source emanating a beam of light; and light collecting means receiving the beam of light emanating from the light source, and collecting the received beam of light on a magneto-optical recording medium. The optical pickup device further includes an optical element formed of an isotropic optical member formed of an isotropic optical medium and an anisotropic optical member formed of an anisotropic optical medium, arranged between the light source and the light collecting means, the isotropic and anisotropic optical members having a boundary surface therebetween serving as a polarization separating surface. The optical pickup device further includes: a photodetector receiving light reflected by the magneto-optical recording medium and directed thereto; and a first diffraction element arranged between the optical element and the photodetector to receive and direct light from the optical element to the photodetector. In the optical pickup device the first diffraction element has a hologram pattern compensating for a wave aberration introduced when the boundary surface refracts light.
The first diffraction element has a hologram pattern compensating for a wave aberration introduced when light refracts passing through a boundary surface of the first and third members. As such, if the optical element is formed of an inexpensive material, reliable signal detection can be achieved. More specifically, in addition to reliable signal detection, the third member, conventionally formed of a particularly expensive material, can be provided at low cost.
In the optical pickup device of the above second aspect the hologram pattern corresponds to a locus of a point H on the first diffraction element satisfying a relationship:
(LHxe2x88x92PH)xe2x80x2=nxcex
wherein L represents a point of light transmitted through a first diffraction element and collected, LH represents an optical path length between point H and a point L, P represents a point of light diffracted by the first diffraction element and collected on the photodetector, PH represents an optical path length between point P and point H, xcex represents a wavelength of a beam of light, n represents an integer, (LHxe2x88x92PH)xe2x80x2 represents a difference between optical path lengths LH and PH with a wave aberration at the boundary surface considered together with one of optical path lengths LH and PH.
The above hologram pattern can be calculated with a computer and it can be formed efficiently on a transparent substrate through photolithography and reactive ion etching (RIE). Consequently, a larger cost reduction can be achieved than when the third member is formed of expensive lithium tetraborate.
The above described present optical pickup device includes a signal detection block detecting a signal only via light diffracted by the first diffraction element.
Since only diffracted light is used to detect a signal, modifying a parameter in designing the first diffraction element allows a spot of collected light to be arranged as desired to facilitate arranging a photodetector in designing the optical pickup device.
In the above described present optical pickup device the first diffraction element is serrated, as seen in cross section.
With the first refraction element serrated as seen in cross section, enhancing a refraction efficiency and increasing an amount of light directed to a photodetector can increase a signal-to-noise ratio to reliably reproduce a signal.
In the above described present optical pickup device between the light source and the optical element on an optical path there may exist a second refraction element. Arranging the second diffraction element allows a stable tracking signal to be output in a 3-beam method. Furthermore in the above described present optical pickup device the optical element includes the third member formed of lithium niobate. In the present invention it is not necessary to combine glass material and birefringent material of the optical element to reduce the difference between the refractive index of the first member and that of the third member. Thus for example such inexpensive materials as lithium niobate and SF2, a product of Schott Group, can be selected and combined.
In the above described present optical pickup device the optical element has opposite sides each provided with a xc2xd wavelength plate of resin.
Arranging a xc2xd wavelength plate as described above allows a light beam emanating from a light source to have polarization in a direction set as desired and furthermore can provide a more inexpensive optical pickup device than when a wavelength plate using a crystal such as quartz is arranged.
In the above described present optical pickup device the light source and the photodetector are arranged in a single package having a translucent window and airtight sealed.
The light source and the photodetector that are arranged in a single, airtight sealed package can have a stable, relative positional relationship maintained for a long period of time to provide a durable optical pickup device.
In the optical pickup device of the above second aspect the isotropic optical member is formed of two isotropic optical media having different indexes of refraction and has a boundary surface formed by the two isotropic optical media and a boundary surface formed by the anisotropic medium and one of the isotropic media so that light reflected by the magneto-optical recording medium can pass through both of the boundary surfaces successively.
Thus in a polarization separating prism at a boundary surface of isotropic optical media light can branch and at a boundary surface of an isotropic optical medium and an anisotropic optical medium polarized light can be separated. As such, in addition to the function of the diffraction element having the hologram pattern, as described above, light-branching and light separation of light reflected from a magneto-optical recording medium can be independently provided. Thus, furthermore reliable photodetection can be provided.